There is currently considerable interest and research into ways to utilize solar energy to help overcome society's present dependence on a diminishing supply of fossil fuels and problem-ridden nuclear fuels. Alternative non-polluting energy sources such as solar energy would be highly desirable. To make solar energy economically feasible, highly efficient radiation absorbing surfaces are needed.
A number of radiation absorbing surfaces are known in the prior art. They may have selective or non-selective characteristics. A selective radiation absorbing surface is one which has high absorptivity and low reflectivity in the solar radiation range and low emissivity and high reflectivity in the thermal infrared region of the electromagnetic spectrum. For a non-selective surface, the absorption and emission properties do not change appreciably in the above two spectral regions.
At least three well-known methods of preparing selective black radiation absorbing surfaces exist in the art. One method involves the coating of a metal base with a thin film that absorbs solar radiation but is transparent to infrared radiation. The high infrared reflectivity of the base provides the low infrared emissivity. A second method requires the coating of an opaque metal or metallic oxide having a high infrared reflectivity but a low solar reflectivity with antireflection layers such that the solar reflectivity is further reduced, thereby enhancing the solar absorption. Finally, a third method of obtaining a selective black absorber is to fabricate a wire-mesh with appropriate dimensions such that solar radiation, but not infrared, is trapped. An example of a selective radiation absorbing surface is disclosed in U.S. Pat. No. 4,148,294 which teaches a solar energy absorbing panel having metal bodies projecting longitudinally out from spaced pores in a substrate.
Non-selective surfaces may be produced by utilizing coatings of paint or enamels. Black paints are preferred and are generally composed of carbon in a binder, or a combination of carbon and calcium phosphate in a binder material. Other black coatings useful as non-selective absorbers are disclosed in U.S. Pat. No. 2,891,879 wherein finely divided aluminum is mixed with an organopolysiloxane and, if desired, further mixed with a suitable solvent such as butyl acetate or toluene. These materials are coated on suitable substrates by a variety of methods, such as spraying, dipping, painting, knife coating, spinning, and printing. A non-selective black, non-shiny and substantially non-reflective surface is disclosed in U.S. Pat. No. 4,138,262 wherein bismuth is first sputtered and then vacuum evaporated onto a flexible plastic substrate followed by a coating of photoactive material. The coated material is then utilized as an imaging film.
Selective radiation absorbing surfaces known in the art suffer from a variety of shortcomings. These are problems of metal grain particle agglomeration, temperature instability, undesired chemical reaction, and high cost of preparation of the coatings. Non-selective absorbing surfaces often suffer from high temperature instability, poor weatherability, and inefficient absorption over an extended wavelength range.
Assignee's U.S. Pat. No. 4,426,437, issued Jun. 17, 1984, discloses imageable articles utilizing the radiation absorbing composite surfaces of the present invention.